IMAGINE you are from an alien civilisation, tasked with collecting a sample from Earth to take back to your planet to look for signs of life. You will only be able to return a small representative sample, which means you will be collecting a small amount of seawater from your visit to this “pale blue dot”.
In a thimbleful of Earth seawater there will be perhaps 10 million viruses, up to a million microbes and certainly no humans. So it’s only a small leap to imagine that, if we ever found life on another planet, viruses would be present too. Why, then, don’t space agencies such as NASA and the European Space Agency look for viruses on other planets?
Before we can begin to think about extraterrestrial viruses, however, we need a good understanding of the viruses on our own planet. This is where things get complicated. Although flu and HIV often make the headlines, we actually know very little about the multitude of viruses that surround us.
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Viruses are crucial for life on Earth. Only in the last few decades have we realised that viruses outnumber all cellular life forms on our planet by at least a factor of 10, probably much more. They have been found in all environments where life has been found, from acidic hot springs to Antarctic glaciers and saturated alkaline brines. All cellular life probably has viruses that coexist with it.
But what exactly is a virus? Scientists can’t agree on a definition. What is agreed is that all known viruses insert their genes into a host cell and reprogram it to make more virus. In some cases the host cell or organism then gets sick. This is why Nobel prizewinning biologist Peter Medawar called viruses “a piece of bad news wrapped up in a protein” – which reflects a view shared by most people. But some, if not most, viruses coexist symbiotically with their hosts, and they are not necessarily bad news. Some may even be good news. Essentially, a virus is a bag of genes, whether good or bad.
What good can viruses do? For a start, they help recycle nutrients in the oceans. They keep organic matter high up in the water column by killing and bursting the microbes they infect. The nutrients released by the burst microbes then provide food for other organisms and maintain the entire food chain, including that of fish and humans.
Moreover, evolution as we know it wouldn’t be possible without viruses, which drive evolution by the introduction of new genes to the organisms they infect. In one of many examples, genes essential for the development of the in the ancestor of all placental mammals. No viruses, no motherhood as we know it. At an American Academy of Microbiology Colloquium in July last year, 24 virologists discussed what life on Earth would look like if there were no viruses. The opinions ranged from no life whatsoever to kilometre-deep slime covering the planet, but we all agreed that life as we know it wouldn’t exist.
If viruses are so important, why do we know so little about them? Two of the main problems are that there are so many different varieties, and they are inherently tricky to study. To address some of these issues, I set up the , together with , founding director of the NASA Astrobiology Institute and discoverer of the hepatitis B virus. It is a forum to discuss where viruses fit in the context of astrobiology – the study of the origin, evolution, distribution and future of life in the universe.
One area of research that this forum has focused on is how to detect viruses. The extracellular parts of viruses – virions – have unique shapes, often with exquisite geometric symmetry. Along with Wolfram Zillig, David Prangishvili and others, I have shown that viruses from hot acidic environments often have virions with highly unusual shapes: lemons, spindles, long filaments with nanoscale terminal claws, or even bottle-shaped. Could their distinctive shapes be used as a virus detection method? This isn’t as straightforward as it might sound. Virions are very small: 200,000 large ones stacked on top of each other would be only 1 millimetre tall. And these shapes can only be seen using an electron microscope, which is unlikely to be put on a spacecraft in the foreseeable future.
A more promising way of detecting viruses might be from their remains. But while fossilised microbes more than 3 billion years old have been found, no virus fossils have ever been reported. Is this because virus fossils can’t be detected? Or are there none? Or has nobody looked?
The detection of virus fossils is a topic I am working on. My student James Laidler and I have shown that , the first step in fossilisation, under simulated hot spring conditions, but that their unique virion shapes were obscured by the deposits in less than a week. However, we were able to detect virus fragments in the silica deposits and are trying to determine unambiguous virus signs, or biosignatures, that could persist in the rock record. This will be critical for searching for extinct viruses both on and off Earth, and will help answer key questions about virus origins: did they evolve relatively recently or are they as old as life itself?
Once we have developed the techniques to find virus biosignatures in ancient rocks on Earth – something we hope to achieve within a year or so – the next step will be to examine known Martian meteorites for virus biosignatures (there are almost 100 kilograms of known Martian meteorites on Earth, some very old). And when samples are , hopefully in 20 to 30 years’ time, I hope that the NASA Virus Focus Group will be involved in both the selection of samples to return and the techniques that will be used to analyse them. To avoid contamination this analysis will probably have to be carried out remotely with no direct human contact, which will also be an interesting engineering challenge.
What do I think the chances are of finding evidence of viruses in Martian samples? It really depends if other life forms ever existed there. If they did, I would be very surprised if there were no viruses. Perhaps the first detected sign of Martian life will be a virus biosignature?
“Perhaps the first detected sign of Martian life will be from a virus”
Viruses are the most numerous, least understood and possibly the oldest biological objects on Earth. We clearly need to learn much more about terrestrial viruses before we can look for them elsewhere. One complication is that the vast majority of Earth virus research is, understandably, on those that cause human disease, even though they are a minuscule proportion of those on this planet. If we are to understand our own virosphere, let alone viruses elsewhere, we must get beyond the “bad news” viruses and appreciate that the vast majority of viruses are an integral part of life.
Given the massive number of Earth-like planets in the universe, with more being detected almost daily, I would be extremely surprised if life, and viruses, were only present on our pale blue dot.
This article appeared in print under the headline “The forgotten extraterrestrials”
